In the field of polymers, one of the most widely manufactured classes of polymers is that encompassing polymerized monovinyl aromatic compounds such as styrenes, alpha-methyl styrenes, and ring-substituted styrenes. Of this class, the most common members are normal polystyrene, and rubber-modified polystyrene--often called high impact polystyrene or HIPS.
In the continuous process of polymerizing styrene monomer into polystyrene and HIPS, a common process of carrying out the polymerization is through the use of polymerization initiators such as t-butyl peroxybenzoate and dibenzyl peroxide. One disadvantage of using such initiators is that a by-product of the polymerization process using these initiators is acid decomposition by-products, such as benzoic acid, which acidic species then react with the initiator and, as a consequence, inhibit further polymerization, when recycled with unreacted monomer back into the polymerization reactors.
One method of handling this problem is that disclosed in Sosa, et al U.S. Pat. No. 4,857,587, the entire disclosure of which is hereby incorporated by reference into this application. In the Sosa patent, recycled unreacted monomer is passed through a recycle treatment vessel to remove acidic species. The vessel would normally contain an adsorbent material such as alumina or clay to remove a major portion of the acid component from the recycle stream.
Other methods of reducing the effect of acid decomposition by-products are disclosed in the two Sosa et al patents, U.S. Pat. Nos. 4,777,210 and 4,861,827, the disclosures of which are hereby incorporated by reference herein. The '210 patent discloses a preinversion reactor to control particle size and the '827 patent teaches the use of initiators which do not decompose into acid by-products during the polymerization process.
The present invention provides an advancement over the known methods of minimizing acid decomposition by-products, which advancement can also be used in conjunction with the above-described patented methods to even further reduce unwanted by-products.